Vibrating meters, such as for example, vibrating densitometers and Coriolis flow meters are generally known and are used to measure mass flow and other information for materials within a conduit. The material may be flowing or stationary. Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit(s), and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flow meter, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or a small “zero offset”, which is a time delay measured at zero flow. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the flow meter lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the time delay between the pick-off sensors. The time delay between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
Meter electronics connected to the driver generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pick-off sensors. The driver may comprise one of many well-known arrangements; however, a magnet and an opposing drive coil have received great success in the vibrating meter industry. Examples of suitable drive coil and magnet arrangements are provided in U.S. Pat. No. 7,287,438 as well as U.S. Pat. No. 7,628,083, which are both assigned on their face to Micro Motion, Inc. and are hereby incorporated by reference. An alternating current is passed to the drive coil for vibrating the conduit(s) at a desired flow tube amplitude and frequency. It is also known in the art to provide the pick-off sensors as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current, which induces a motion, the pick-off sensors can use the motion provided by the driver to induce a voltage. The magnitude of the time delay measured by the pick-off sensors is very small; often measured in nanoseconds. Therefore, it is necessary to have the transducer output be very accurate.
FIG. 1 illustrates an example of a prior art vibrating meter 5 in the form of a Coriolis flow meter comprising a sensor assembly 10 and a meter electronics 20. The meter electronics 20 is in electrical communication with the sensor assembly 10 to measure characteristics of a flowing material, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information.
The sensor assembly 10 includes a pair of flanges 101 and 101′, manifolds 102 and 102′, and conduits 103A and 103B. Manifolds 102, 102′ are affixed to opposing ends of the conduits 103A, 103B. Flanges 101 and 101′ of the prior art Coriolis flow meter are affixed to opposite ends of the spacer 106. The spacer 106 maintains the spacing between manifolds 102, 102′ to prevent undesired vibrations in the conduits 103A and 103B. The conduits 103A and 103B extend outwardly from the manifolds in an essentially parallel fashion. When the sensor assembly 10 is inserted into a pipeline system (not shown) which carries the flowing material, the material enters sensor assembly 10 through flange 101, passes through the inlet manifold 102 where the total amount of material is directed to enter conduits 103A and 103B, flows through the conduits 103A and 103B and back into the outlet manifold 102′ where it exits the sensor assembly 10 through the flange 101′.
The prior art sensor assembly 10 includes a driver 104. The driver 104 is affixed to conduits 103A and 103B in a position where the driver 104 can vibrate the conduits 103A, 103B in the drive mode, for example. More particularly, the driver 104 includes a first driver component 104A affixed to the conduit 103A and a second driver component 104B affixed to the conduit 103B. The driver 104 may comprise one of many well-known arrangements such as a coil mounted to the conduit 103A and an opposing magnet mounted to the conduit 103B.
In the present example of the prior art Coriolis flow meter, the drive mode is the first out of phase bending mode and the conduits 103A, 103B are selected and appropriately mounted to inlet manifold 102 and outlet manifold 102′ so as to provide a balanced system having substantially the same mass distribution, moments of inertia, and elastic modules about bending axes W-W and W′-W′, respectively. In the present example, where the drive mode is the first out of phase bending mode, the conduits 103A and 103B are driven by the driver 104 in opposite directions about their respective bending axes W-W and W′-W′. A drive signal in the form of an alternating current can be provided by the meter electronics 20, such as for example via pathway 110, and passed through the coil to cause both conduits 103A, 103B to oscillate. Those of ordinary skill in the art will appreciate that other drive modes may be used by the prior art Coriolis flow meter.
The sensor assembly 10 shown includes a pair of pick-offs 105, 105′ that are affixed to the conduits 103A, 103B. More particularly, first pick-off components 105A and 105′A are located on the first conduit 103A and second pick-off components 105B and 105′B are located on the second conduit 103B. In the example depicted, the pick-offs 105, 105′ may be electromagnetic detectors, for example, pick-off magnets and pick-off coils that produce pick-off signals that represent the velocity and position of the conduits 103A, 103B. For example, the pick-offs 105, 105′ may supply pick-off signals to the meter electronics 20 via pathways 111, 111′. Those of ordinary skill in the art will appreciate that the motion of the conduits 103A, 103B is generally proportional to certain characteristics of the flowing material, for example, the mass flow rate and the density of the material flowing through the conduits 103A, 103B. However, the motion of the conduits 103A, 103B also includes a zero-flow delay or offset that can be measured at the pick-offs 105, 105′. The zero-flow offset can be caused by a number of factors such as non-proportional damping, residual flexibility response, electromagnetic crosstalk, or phase delay in instrumentation.
In many prior art fluid meters, the zero-flow offset is typically corrected for by measuring the offset at zero-flow conditions and subtracting the measured offset from subsequent measurements made during flow. While this approach provides an adequate flow measurement when the zero-flow offset remains constant, in actuality the offset changes due to a variety of factors including small changes in the ambient environment (such as temperature) or changes in the piping system through which the material is flowing. As can be appreciated any change in the zero-flow offset results in an error in the determined flow characteristics. During normal operations, there may be long periods of time between no-flow conditions. The changes in the zero-flow offset over time may cause significant errors in the measured flow.
The present applicants have developed a method for determining and correcting for changes in the zero-flow offset during flow, which is described in U.S. Pat. No. 7,706,987 entitled “In-Flow Determination Of Left And Right Eigenvectors In A Coriolis Flowmeter” and is hereby incorporated by reference. This so-called “Direct Coriolis Measurement” (DICOM) used in the '987 patent explains that if two or more drivers are used rather than the typical single driver system, the left and right eigenvectors of the Coriolis flow meter system can be determined. In the physical sense, the right eigenvectors determine the phase between response points (pick-offs) when a particular mode is excited. The right eigenvectors are the values typically measured and determined in vibrating flow meters, such as the prior art flow meter 5. The left eigenvectors determine the phase between drivers that optimally excite a particular mode. Without a zero-flow offset, these two phases are the same. Consequently, as outlined in the '987 patent, if the left and right eigenvectors can be determined, the zero-flow offset can be distinguished from the fluid flow.
Although DICOM allows for increased accuracy in flow measurements by allowing in-flow determination of the zero-flow offset, the present applicants have discovered that the DICOM requires collocated sensor components. Although the '987 patent describes the use of collocated sensor components, in actuality, the '987 patent utilizes two separate and distinct driver sensor components and two separate and distinct pick-off sensor components. The '987 patent attempts to position the driver and pick-off sensor components directly across from one another on the flow conduit to provide collocation. However, because the driver and pick-off sensor components are individually attached to the flow conduits 103A, 103B, precise collocation is impractical and even a small misplacement can result in errors propagating throughout the flow measurement.
U.S. Pat. No. 6,230,104, which is assigned on its face to the present applicants, discloses a combined driver and pick-off sensor. The combined driver and pick-off sensor disclosed in the '104 patent can be used to reduce the number of sensor components, which reduces the wiring and consequently, the cost. Additionally, the combined driver and pick-off sensor can be used to perform DICOM. However, due to the configuration of the combined sensor component disclosed in the '104 patent, measurements are complex and require an excessive amount of power. Further, the configuration disclosed in the '104 patent is easily rendered inaccurate. The '104 patent uses the same coil to apply the drive signal and receive the pick-off signal. This dual use of the coil requires a complex separation of the back electromotive force (back-EMF), which is the desired velocity measurement, from the measured transducer voltage applied by the drive signal. The determination of the back-EMF with the combined sensor component shown in the '104 patent requires at least two compensations. The back-EMF can be characterized by equation (1).
                                          V            bEMF                    =                                    V              total                        -            Ri            -                          L              ⁢                                                ⅆ                  i                                                  ⅆ                  t                                                                    ⁢                                  ⁢                  Where          ⁢                      :                          ⁢                                  ⁢                                            V              bEMF                        ⁢                                                  ⁢            is            ⁢                                                  ⁢            the            ⁢                                                  ⁢            back            ⁢                          -                        ⁢            EMF                    ;                ⁢                                  ⁢                                            V              total                        ⁢                                                  ⁢            is            ⁢                                                  ⁢            the            ⁢                                                  ⁢            total            ⁢                                                  ⁢            measured            ⁢                                                  ⁢            pick            ⁢                          -                        ⁢            off            ⁢                                                  ⁢            voltage                    ;                ⁢                                  ⁢                              Ri            ⁢                                                  ⁢            is            ⁢                                                  ⁢            the            ⁢                                                  ⁢            resistive            ⁢                                                  ⁢            load                    ;          and                ⁢                                  ⁢                  L          ⁢                                    ⅆ              i                                      ⅆ              t                                ⁢                                          ⁢          is          ⁢                                                            ⁢                                                          ⁢          the          ⁢                                          ⁢          inductive          ⁢                                          ⁢                      load            .                                              (        1        )            
The resistive load varies with temperature, thereby requiring on-line resistance calculation. Errors in this compensation affect both drive stability and flow measurement. Further, the resistive load is typically larger than the other terms in equation (1). Consequently, even small errors in the resistive load can translate to large flow errors. The inductive load is typically much smaller than the resistive load, but small errors here still become significant flow measurement offsets.
Therefore, as can be appreciated, the combined driver and pick-off sensor disclosed in the '104 patent does not provide a suitable solution. There exists a need in the art for a combined driver and pick-off sensor that is collocated and can determine measurements with reduced complexity. The embodiments described below overcome these and other problems and an advance in the art is achieved.